THEMATIC MAPS

WHAT IS A THEMATIC MAP?

There are three classes of maps: 1) general reference maps which show the locations of a variety of features; 2) thematic maps which show the distribution of a single attribute (characteristic) or the relationship between several attributes, and; 3) charts which are used primarily for navigational purposes.

Thematic maps can cover a variety of characteristics from soil types to population density. It is the cartographers responsibility to make sure that the map shows the correct distribution or the relationship between the various attributes.

WHEN DO WE USE THEMATIC MAPS?

Thematic maps can show not only the distribution of a single attribute, such as the result of a presidential election by state, but they can also show the relationship between several different attributes. For instance, a thematic map could show the results of that same presidential election with the number of votes divided according to the gender or age group of the voters.

Some maps that deal with a single attribute such as population, may not necessarily be thematic maps. If the map shows the actual location where the people live, it would be a general reference map. A map showing the distribution of that same population would be a thematic map.

HOW DO WE SCALE A THEMATIC MAP?

Until fairly recently, thematic maps were usually made with a small scale, because the data was rather coarse, and it was more important to show the basic distribution pattern than the map location for the data. In recent years, however, better data has become available and thematic maps are being made with a larger scale to show more accurate spatial information.

WHAT MAKES A GOOD THEMATIC MAP

When designing a thematic map, a cartographer must be careful to portray the data on the map so that it will be easy for the audience to use and understand. This is accomplished mainly with the marks and symbols that the cartographer uses to represent the data. The designer should also give an adequate locational base for the map.

AN INTRODUCTION TO CHOROPLETHIC MAPPING

 

BACKGROUND INFORMATION

A choropleth is a map that shows a statistical surface with symbols or colors, that coincide with the surface region that the information was collected from. A quantity mapped choroplethically is generally some type of geographical average referring to the whole of an observation area, and a symbol, usually a grey shading or color, representing a quantity is placed over that area. There are two types of choropleths, classed and unclassed. A classed choropleth portrays varying statistics that occur within specified boundaries. For example, a classed choropleth might portray an enumeration district within a county, city, or state. A classed choropleth follows a pattern of spatial organization of information without showing trends salient to that information, and avoiding specific numbers from that data, it uses numerical averages to portray information.

An unclassed choropleth is designed to show statistical data in a multi-colored value moving from smallest to largest, using lighter colors to portray smaller values and using progressivly darker shades to portray progressivly greater values. Examples of an unclassed choropleth might include: how many head of cattle ar being sold per far per county within the state of Nebraska. Another example might be what the average rainfall in inches was for the farmland of Colorado per county.

 

The Utility of Choropleths

Choroleths are maps that generalize specific statistical information, into averages that can be understood graphically. Although choropleths are generalized they do provide an excellent overview of information distribution.

 

Essentials of Choroplethic Mapping

Choropleths display information in three primary elements. They are: 1) size and shape. 2) number of classes shown. 3) class limits methodologically.

Examples of Choroplethic Mapping Elements

A good color pattern should progress from lighter to darker portraying smaller statistical units to larger, this may include monochromatic or color applications as both show a progression of color differentiation. The size and shape of a choropleth should form a pattern that is discernable, the number of classes is delimited by the amount of data being represented.

Click here to a bad example of a choropleth

This is a bad example of what a choropleth should ideally be as it has an unnatural color progression and does not show any trends in the desired information when there clearly should be.

Click here to see an example of a better choropleth This is a better example of a choropleth as it does provide a better sense of pattern associated with the use of color to display information. However it does not quite meet all of the requirements that are necessary to the production of a quality choropleth. The problem with this choropleth is that it doen not move from lighter to darker with its color progressinon and this causes some dificulty in the interpretation of the information.

This is an example of a color progression that is an acceptable method when using color to show an incremental change in the statistical data. The yellow is the lightest and lowest valued in data and the progressively darker shades show higher statistical values.

As you can see the previous two examples are gradual, subtle changes in color and are easy to follow from least to greatest. This is the optimal configuration for a choropleth and should be the goal of the cartographer as he/she crafts their choropleth.

Choropleths can be an extremely useful tool when trying to understand more about statistical data within a geographic area.

Brought to you by the folks in Dr. Michael Peterson's Cartography class. Goodbye and happy choroplething!!!!

Symbolizing: Basic Design Principles

Graphic elements, controls and components


Goal: present geographic information using graphic media in such a manner to communicate the message effectively and efficiently.

The Model: Robinson, and coauthors describe map design as an interaction ­ graphic elements are the visual variables which can be manipulated to achieve the components within the bounds of the controls. This is a paraphrase of Bertin's Semiologie Graphique. Dent has part of it, MacEachern develops it further.

Primary Graphic Elements

("Symbol dimensions" in Dent)

Graphic Components

  • Clarity and Legibility
  • Visual Contrast
  • Visual Balance
  • Figure ­ Ground
  • Hierarchy
    •

    Five Controls of graphic design:

  • Objective
  • Reality
  • Scale
  • Technical limits
  • Audience
    •

    Defining a thematic map: geometry and attributes

    Objectives of lecture

     


    Cartography: the art and science of making maps (International Cartographic Association)
    "A map is a graphic representation of the milieu." (cultural and physical environment)

     

    [a definition so broad that it doesn't help much...]

    Varieties of maps...

    Each of these is a "genre", a formalized arrangement of expectations about how the signs work. [The tools of thematic cartography can be used to build a reference map with many "layers".]

    Two 'schools' of cartography:

     

     

    Mapping as a process of abstraction:

    The "Real World", in all of its diversity is viewed with a specific purpose in mind.
    A system of symbols portray the information (originally graphics symbols, now also data structures)
    Transformation that restructures source material to make a map:
    The World is compressed - represented by the map.
    (from Erwin Raisz, 1962, Principles of Cartography, chapter 3)

    1 Maps are drawn in a predetermined scale.
    2 Maps are selective, based on purpose of map.
    3 Maps emphasize certain of selected features (themes)
    4 Maps are symbolized.
    5 Maps are generalized. Intricate detail is simplified.
    [ 6 Maps are lettered, titled and labelled.
    7 Maps are usually related to a reference system] List applies to databases?

    Ways to look at the World:

    The complete mapping process

  • A process of communication: Concepts, facts transmitted through the map
  • Data collection (followed by selection, processing, transformation)
  • Map construction (encode the information for a particular purpose)
  • Map Use ­ the encoded message does no good unless it can be used...
  • Map Reading ­ deciphering symbols (relating to intended message)
  • Map Analysis ­ construct spatial patterns and relationships
    (a process of structured map reading, measurement...)
  • Map Interpretation ­ link spatial form & patterns to causation (process)
    •
  • BUT this is nowhere near as linear as it sounds!
  • Map makers, readers, users are all surrounded by a lot of pre-existing social cultural arrangements that communicate meaning...
  • lots of map use is directly by the map maker, an aid to thinking, not arms-length communication (in a more private realm: MacEachern Exploration, Confirmation, Synthesis, Presentation)
  • Map as inventory, repository of spatial facts and arrangements (database)
  • Communities of Practice develop (disciplines) to set expectations that don't need to be communicated... The insiders KNOW what will be on the map. Outsiders beware.
    •

    • Field of Semiotics (the study of signs-symbols) distinguishes

    Multiple versions of this triangle with each connection emphasized...

    Basic agreement:

    Good maps:

  • Is a map just to be judged by its correspondence to the world (according to whose rules?)
  • Or is a map just an element of technology that does a "job"?
  • Or does this difference sound too much like splitting hairs?
    •

    Perhaps the most important function of a map:

    As an "inscription", it makes the world portable... (you can move it around without changing what you wrote down)

     

    Components of map information:

    Time, Space, & Attributes
    Role of "Reference Systems" to position in Time (calendar, clock), Space (geodetic position), and Theme

    Attributes are the "content" delivered in the wrapping of their reference to time and geometry.

    [also called "theme": thematic 'overlay' adds content to the base geometry]

    Many relationships can be formed (measured, represented, extracted ...)

    Location1 => L2 distance, bearing, geometric relationships
    L1 (Attribute1, A2, ...) relationships between attributes at one "place" [and one time]
    L1 (A1) => L2 (A1) relationships between attributes at different places/times

    Topographic vs. thematic maps

    Types of thematic map

    Point maps

    1. Simple point maps: –All points are identical and have same value –Can indicate precise location of objects or phenomena –Can indicate general distribution of collection of objects 
    2. Complex point maps: –Different types or sizes of symbols for different phenomena

    Proportional symbol maps

    1. Size of symbol is related by a defined scale to the magnitude or other significance of the phenomenon being mapped 
    2. May be area, height, diameter, etc. of symbol 
    3. Symbols may be circles, squares, other geometric shapes, graphs or even icons •Graphs can be used to add further (statistical) information 

    Area maps

    Flow line maps

    Isoline maps

    Gridded maps

    Cartograms

    14. THEMATIC MAPS (Point Data)

    1. Qualitative Thematic Maps

    These form a variety of maps, and differ from general or topographic maps in detailing the distribution of one category of data such as those below; they deal with nominal data and hence use qualitative visual variables for the map elements, e.g. different types of forest cover or religions.

    General design goals

    2. Quantitative Thematic Maps Point Data

    Dot Maps

    Design factors
    The choice of unit value and dot size are critical: the desirable density will just begin to merge at the greatest density (can still be counted), but give a good visual impression of distribution

    Possible results:

    Dot techniques break down when: Under these conditions, vary the size of the point symbol instead of the number of them relating to a point location. This is known as the graduated symbol technique.

    3. Graduated Symbols

    a. Bars

    b. Circles

    Figure 10-1 : Graduated Circles  (click on image to enlarge)

    Other Shapes
    Other geometric shapes can be used such as triangles and squares, or any designed iconic shape - awkward with manual mapping but variable sizes can be easily achieved with  software.

    Figure 10-2 Utilization of various geometric shapes (click on image to enlarge)

    c. Range Graded Circles

    d. Segmented Symbols

    e. Volumetric Symbols

    Figure 10-3:  Graduated symbols with size changes proportional to value.
     

    15. THEMATIC MAPS (Line & Area Data)

    1. Line Data

    a. Graduated Line Symbols

    This follows the same principle as the graduated bar, but line symbols are changed in width instead of height. Thickness is used to imply 'flow' or 'volumes', particularly where movement is involved, e.g. trade, river flow, or traffic. Representative widths are shown in the legend; one can use broken lines for the smallest values.

    Figure 10-4 : Graduated line map (click on image to enlarge)

    b. Isarithms

    An isarithm is a line joining equal values of some variable. These are usually derived or interpolated from point measurements. The main examples are climatological, e.g. isobars, isohyets, isotherms, as well as isobaths, lines of equal water depth, from which contours were first developed. Like contour lines, an interval between adjacent lines is established by the user, based on total range and scale.

    Figure 10-5 : Example of an isarithmic map using lines. (Source: The Globe and Mail)

    2. Area Data

    Area techniques depict data values for areas, but which may have been interpolated from point values. Care should be taken in selecting a technique based on whether the value is an absolute total (e.g. total number of people) versus a ratio or density (e.g. population density).
    Three main choices are available to map a statistical surface using quantitative attribute values:

    a. Isarithmic = 'equal values'

    This is the same as the isarithmic line technique, but with value ranges filled in with colours, analogous to contours and hypsometric tints. Choice of colours may be selected according to the feature being mapped, e.g. blue & red for temperature, yellow for sunshine. Increased tones or chromas are being used for higher ranges; colour schemes are often single hue (chromatic) but may use 2 hues (bichromatic) with diverging chroma away from the mean (as with hot and cold temperatures).
    This type of map is most often used to display climatic data as in Figure 10-6.

    Figure 10-6 :  Isarithmic map showing temperature   (Source:  The Globe and Mail)

    b. Dasymetric = measure of density

    Dasymetric maps depict intensities rather than 'counts', e.g. %, ratios;
    They use the same type of data as Choroplethic, but involve some analysis beyond the administrative districts; i.e. do not assume homogeneity within districts. Note that lines which may look the same as in the isarithmic technique but do not 'have' values ... values can go from high to low, without going through intermediate values as in the isarithmic technique.  See the figure below.

    Figure 10-7 : Dasymetric map (Source:  The Globe and Mail)

    c. Choroplethic = magnitude at place

    This is used to map intensity or ratio data, not absolute numbers, since these would be biased by a large data collection area. Tones or chroma are used as with other techniques to depict values. Choropleth maps are very common as much data is collected by census districts or larger units.

    The results take on very different looks depending on the data classes devised, since the original values cannot all be retained against the principles of generalization.  Data collection is summarized over areas and broken down into like units. When mapped, the units appear uniform ending at the boundaries;where they may imply sudden change, but not be meaningful.  In this type of map the magnitude of the statistics are symbolized within the boundaries of divided units -e.g. provinces, or political districts.

    Design of Choropleth Classes
    Class Schemes include the following:

    The 'default' scheme is 'equal steps' but where data are skewed or normally distributed, one gets too many values in one class or a few classes.  The other schemes attempt solutions for this common situation (see sample statistics below).

    General Goals

    Table 10-8 : Sample data for 20 areal units
    e.g. % of population with post secondary education:  average = 5.0 median = 7.4
     
    1.9 
    4.4 
    7.6 
    10.5 
    2.7 
    4.8 
    7.7 
    14.1 
    3.3 
    4.9 
    7.9 
    19.1 
    3.4 
    5.3 
    9.0 
    22.6 
    3.5 
    7.2 
    10.4 
    39.8 

     

    Table 10-9 : Creating class boundaries using sample data from Table 10-8
     
     
    Class range 

     
    Freq-
    uency
    (F)

     
    Class
    Range
     F 
    Class range
     F 
    Class range
     F 
    Class range
     F 
    Equal steps:
    0-8
    13
    8-16
    4
    16-24
    2
    24-32
    0
    32-40
    1
    Geometric:
    0-4
    5
    4-8
    8
    8-16
    4
    16-32
    1
    32-64
    1
    Quantiles:
    0-3.4
    4
    3.4-5
    4
    5-7.8
    4
    7.8-12
    4
    12-40
    4
    Natural breaks:
    0-4
    5
    4-6
    4
    6-12
    7
    12-25
    3
    25-40
    1

    d. Isometric diagrams

    One problem with choropleth maps is we are forced to class data.  How should we design classes, how much information is lost and how can we show more than 6 or 8 classes?

    One solution is to use height instead of colours to represent the collection units; these become isometric 'diagrams'; each unit is 'raised' vertically by the appropriate amount.
    Manually this was a very tedious process requiring transformation of the base map.

    Manual method:

    3. Cartograms

    In some ways, a cartogram is a 'cross' between a map and a diagram: it has no consistent scale according to cartesian distance, but is consistent based on some other geographic variable: